The Relevant Sense of “Experience”
In Section 1 above, it was noted that a posteriori justification is said to derive from experience and a priori justification to be independent of experience.
To further clarify this distinction, more must be said about the relevant sense of “experience”.
There is NO widely accepted specific characterization of the kind of experience in question.
Philosophers instead have had more to say about HOW NOT TO characterize it.
There is broad agreement, for instance, that experience should not be equated with sensory experience, as this would exclude from the sources of a posteriori justification such things as memory and introspection.
(It would also exclude, were they to exist, cognitive phenomena like clairvoyance and mental telepathy.)
Such exclusions are problematic because most cases of memorial and introspective justification resemble paradigm cases of sensory justification more than they resemble paradigm cases of a priori justification.
It would be a mistake, however, to characterize experience so broadly as to include any kind of conscious mental phenomenon or process; even paradigm cases of a priori justification involve experience in this sense.
This is suggested by the notion of rational insight, which many philosophers have given a central role in their accounts of a priori justification.
These philosophers describe a priori justification as involving a kind of rational “seeing” or perception of the truth or necessity of a priori claims.
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There is, however, at least one apparent difference between a priori and a posteriori justification that might be used to delineate the relevant conception of experience (see, e.g., BonJour 1998).
In the clearest instances of a posteriori justification, the objects of cognition are features of the actual world which may or may not be present in other possible worlds.
Moreover, the relation between these objects and the cognitive states in question is presumably causal.
But neither of these conditions would appear to be satisfied in the clearest instances of a priori justification.
In such cases, the objects of cognition would appear (at least at first glance) to be abstract entities existing across all possible worlds (e.g., properties and relations).
Further, it is unclear how the relation between these objects and the cognitive states in question could be causal.
While these differences may seem to point to an adequate basis for characterizing the relevant conception of experience, such a characterization would, as a matter of principle, rule out the possibility of contingent a priori and necessary a posteriori propositions.
But since many philosophers have thought that such propositions do exist (or at least might exist), an alternative or revised characterization remains desirable.
All that can be said with much confidence, then, is that an adequate definition of “experience” must be broad enough to include things like introspection and memory, yet sufficiently narrow that putative paradigm instances of a priori justification can indeed be said to be independent of experience.
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